1. Field of the Invention
The invention relates to a method of manufacturing pulverulent polymaleic anhydride (poly-MA) by radical polymerization of maleic anhydride (MA) in an organic solvent.
2. Discussion of the Background
The copolymerization of MA with olefins has long been known. However, the first reports of polymerization of MA alone appeared in 1961 (See 1962, Makromol. Chemie, 53:33), with UV irradiation or with initiation by means of organic peroxides.
In Eur. Pat. No. 0 009 171, polymerization of MA in acetic anhydride with hydrogen peroxide was proposed. Because the anhydride groups are hydrolyzed at the same time, this method produces polymaleic acid directly. If the acetic anhydride is replaced by toluene, only very dark polymers are obtained.
U.S. Pat. No. 3,708,436 discloses polymerization of MA in high-percent solutions in toluene, with the aid of benzoyl peroxide. In this method, poly-MA is obtained as a solid mass. Yields are about 35-60%.
According to Brit. Pat. No. 1,193,146, MA is polymerized in 15-50% solutions in toluene with the aid of benzoyl peroxide. Here the polymer is obtained as a black tar. The product can be crystallized by reprecipitation.
In U.S. Pat. No. 3,919,258, MA is polymerized in xylene at 120.degree.-145.degree. C., using di-tert-butyl peroxide in the amount of 15-40% (based on the MA). The poly-MA is isolated as a clear brown liquid, from which a cream-colored powder can be obtained by reprecipitation.
According to Ger. Pat. No. 1,162,083, MA can be polymerized in dilute solutions in inert solvents, with benzene being the preferred solvent. The products are obtained as finely dispersed powders. They are filtered and are purified by ether extraction. The radical-forming agent primarily used in the polymerization is dicyclohexyl percarbonate. Dibenzoyl peroxide is less reactive in the system described, and even with activation by cobalt oleate still gives relatively low yields of poly-MA.
The known methods of polymerization thus have the following characteristics:
(a) Poly-MA is obtained only as a solid mass or a liquid, when prepared in toluene solution;
(b) Polymerization in xylene requires high initial concentrations, with the poly-MA being obtained initially as a liquid, from which a powder can be produced by reprecipitation;
(c) With polymerization in benzene using low concentrations, poly-MA can be obtained as a powder. However, this powder must be purified by ether extraction.